A linear RF amplifier is an amplifier that increases the power of a modulated carrier signal while preserving both the amplitude and phase modulation components of the the signal. Frequency-modulation (FM) signals have constant amplitude and thus do not require linear amplification. On the other hand, digital transmission systems may require both amplitude modulation (AM) and phase modulation (PM) of the transmitted signals. Linear amplifiers are necessary to transmit those signals.
Linear RF amplifiers typically have inherent defects, including amplitude distortion and AM-to-PM conversion, which may cause undesired interference called splatter, to adjacent channel signals. Negative feedback, as well as careful amplifier design, is required to minimize the level of splatter generated.
Linear RF amplifiers commonly use feedback to minimize the splatter. For stability (i.e., freedom from oscillation) a feedback loop must have a gain/phase versus frequency characteristic such that the open loop gain (i.e., the gain of the entire feedback loop with the loop broken at one point) is less than one at all frequencies where the phase shift is more than 180 degrees different from that at midband. Previous RF feedback loops have used a very high Q (i.e., the merit factor of the loop) resonant circuit at the radio-frequency to achieve this condition. This approach has several disadvantages, including large size, large RF gain needed, and the limited amount of open loop gain possible.
Stability of the feedback loop is a major concern. The baseband portions of the loop can be made to have well controlled phase characteristics, but the RF portion may have large, unpredictable phase shifts due to resonant circuits, varactor effects of supply voltage variations, AM-to-PM conversion, thermal effects and aging effects. These effects substantially reduce the phase margin for stability of the loop, and may even cause oscillation of the amplifier.